The Evolution of Geographic Variation in Birdsong

Evolutionary biologists have a long‐standing interest in how organisms vary geographically. This interest is motivated in part by recognition of a relationship between geographic variation and the process of speciation. Traits that vary over a given species’ range may serve as neutral, noncontributing indicators of the early stages of divergence, for example as different populations adapt to distinct environments and undergo corresponding genotypic and phenotypic divergence (Schluter, 2000). In other cases, traits that vary geographically might contribute to the speciation process. Primary examples of such traits are mating ornaments and displays which, in many animals, are centrally involved in mate recognition and mate selection (Andersson, 1994; Boughman, 2001; Foster, 1999; Panhuis et al., 2001; Wells and Henry, 1998; West‐Eberhard, 1983). Geographic divergence of mating signals can, under particular circumstances, facilitate assortative mating, reproductive isolation, and thus continued divergence among populations (Irwin et al., 2001; Lachlan and Servedio, 2004; Liou and Price, 1994; Pa yne et al. , 2000; Ptacek, 2000 ; Slabbekoo rn and Smith, 2002a). Divergence of mating signals and mate recognition systems is increasingly recognized as an important factor in speciation (Ryan, 1986). Studies of vocal signals in birds offer potentially useful opportunities for empirical tests of the relationships among geographic signal divergence, reproductive isolation, and speciation (reviewed by Edwards et al., 2005). Many bird vocalizations express significant geographic variation, a phenomenon that can be attributed largely to the tendency for many birds to